The present invention is directed to stationary contact straps for circuit breakers, and in particular for molded case circuit breakers.
Molded case circuit breakers (hereafter sometimes referred to as "MCCBs") are utilized as circuit protection devices to interrupt current flow in an electric power line upon detection of conditions which do not fall within predefined parameters. For example, if a short circuit occurs within a power line, current flow can increase dramatically above that which is expected during normal operating conditions. The MCCB has an electrically insulative housing which contains at least one stationary and one moving main contact. During normal operational conditions, the main contacts are in a closed, abutting relationship which allows current flow continuity between the contacts and through the power line. When the main contacts separate, current flow is interrupted. Such interruption may occur when the MCCB trip unit which monitors power in the power line detects an operating condition which is not within predefined operational parameters. Alternatively, a user may selectively interrupt power in the power line by manually opening the main contacts by way of an external operating mechanism. General construction and operation of a molded case circuit breaker is set forth in U.S. Pat. No. 4,513,267 ("the '267 patent"), the entire contents of which is hereby incorporated herein by reference as if it were fully set forth herein. The '267 patent is owned by the assignee of the present invention, Siemens Energy & Automation, Inc.
As shown and described in U.S. Pat. No. 4,513,267, the stationary contact is connected to a stationary contact strap in order to provide electrical continuity between a power line and the contact. In order to increase the speed in which the MCCB main contacts separate, and thus decrease the amount of power which passes through the circuit breaker after initiation of disruption, the '267 patent describes an claims a stationary contact strap which splits input current flow among first and second exterior arms defined by the strap, which are outboard an interior arm that is connected to the exterior arms. The arms are defined in a contact strap by a generally U-shaped slot formed in the strap by a punching, milling or other known material removing operation. Current does now flow in the slot, because there is no current-carrying material. In this manner, current flow through the interior arm is channeled in a direction opposite to current flow in the moveable contact arm, so as to create a repulsive "blow off" effect during transient current surges. The same concept is embodied in the stationary contact line strap shown in U.S. Pat. No. 4,950,853 ("the '853 patent").
While the stationary contact strap of the '267 patent provides excellent blow off characteristics, the u-shaped slot defined in the strap is relatively difficult to form by metal punching techniques, due to its large size, and tends to wear out tooling quicker than would otherwise be desirable. It would also be desirable to have a greater cross-sectional area available for reduced steady state current flow density in the stationary contact strap than is possible with the design of the '267 patent, while maintaining the blow off characteristics possible with that earlier design.
It is an object of the present invention to create a stationary contact strap with increased steady state current flow cross section for lower current densities and greater heat transfer capabilities than known stationary contact straps, yet which provides repulsive blow off during overcurrent conditions in the manner of those known designs.
It is an object of the present invention to create a stationary contact strap which is easier to form than known stationary contact straps, yet which provides repulsive blow off during overcurrent conditions in the manner of those known designs.